1. Field of the Invention
The present invention relates to a process for fabricating a crystalline semiconductor for use in thin film devices such as thin-film insulated-gate field-effect transistors (hereinafter referred to simply as xe2x80x9cthin film transistorsxe2x80x9d or xe2x80x9cTFTsxe2x80x9d). The present invention also relates to a process for fabricating a semiconductor device using the same.
2. Prior Art
Thin films of crystalline silicon semiconductor for use in thin film devices such as TFTs known heretofore have been fabricated by crystallizing an amorphous silicon film formed through plasma CVD (chemical vapor deposition) or thermal CVD, using an apparatus such as an electric furnace maintained at a temperature of not lower than 600xc2x0 C. for a duration of 12 hours or longer. Thin films of crystalline silicon semiconductor having sufficiently high quality (for example, an excellent field effect and a high reliability) are available only after subjecting the amorphous film to a heat treatment for a still longer duration.
However, those prior art processes for obtaining thin films of crystalline silicon semiconductor suffer various problems yet to be solved. One of the problems is the low throughput which increases the process cost. For instance, if a duration of 24 hours is required for the crystallization step, 720 substrates must be processed at a time considering that preferably, the substrates each consume 2 minutes of process time. However, the maximum number of substrates an ordinary tubular furnace can treat at a time is limited to 50; in a practical treatment using only one apparatus (reaction tube), it has been found that a single substrate consumes 30 minutes to complete the treatment. In other words, at least 15 reaction tubes are necessary to complete the reaction per single substrate in 2 minutes. This signifies an increase in investment cost and therefore an increase of the product price due to a too large depreciation for the investment.
The temperature of the heat treatment is another problem to be considered. In general, a TFT is fabricated using a quartz glass substrate comprising pure silicon oxide or an alkali-free borosilicate glass substrate such as the #7059 glass substrate manufactured by Corning Incorporated (hereinafter referred to simply as xe2x80x9cCorning #7059 substratexe2x80x9d). The former substrate has such an excellent heat resistance that it can be treated in the same manner as in a conventional wafer process for semiconductor integrated circuits. However, it is expensive, and, moreover, the price increases exponentially with increasing the area of the substrate. Thus, at present, the use of quartz glass substrates is limited to TFT integrated circuits having a relatively small area.
On the other hand, alkali-free borosilicate glass substrates are inexpensive as compared to those made of quartz glass, however, they have shortcomings with respect to their heat resistance. Since an alkali-free glass substrate undergoes deformation at a temperature in the range of from 550 to 650xc2x0 C. and more particularly, since a readily available material initiates its deformation at a temperature not higher than 600xc2x0 C., any heat treatment at 600xc2x0 C. causes an irreversible shrinkage and warping to form on the substrate. These deformations appear particularly distinctly on a substrate having a diagonal length of more than 10 inches. Accordingly, it is believed requisite to perform the heat treatment on a silicon semiconductor film at a temperature of 550xc2x0 C. or lower and for a duration of within 4 hours to reduce the entire process cost.
In the light of the aforementioned circumstances, an object of the present invention is to provide a process for fabricating a semiconductor which can resist to a heat treatment conducted under the above conditions, and to provide a process for fabricating a semiconductor device using such semiconductors.
An embodiment according to the present invention provides a process for fabricating a semiconductor, which is characterized in that it comprises forming an insulator coating on a substrate; exposing said insulator coating to a plasma; forming an amorphous silicon film on said insulator coating after its exposure to said plasma; and crystallizing said silicon film by photo annealing said silicon film and/or heat treating said silicon film in the temperature range of from 400 to 650xc2x0 C. or at a temperature not higher than the glass transition temperature of the substrate.
Another embodiment according to the present invention provides a process for fabricating a semiconductor, which is characterized in that it comprises forming an insulator coating on a substrate; selectively coating said insulator film with a masking material; exposing said substrate to a plasma; forming an amorphous silicon film on said insulator coating after exposing the substrate to said plasma; crystallizing said silicon film by photo annealing said silicon film and/or heat treating said silicon film in the temperature range of from 400 to 650xc2x0 C. or at a temperature not higher than the glass transition temperature of the substrate; and selectively etching said silicon film.
Furthermore, still another embodiment according to the present invention comprises fabricating a thin film transistor which is characterized in that it comprises forming an insulator coating on a substrate; selectively coating said insulator coating with a masking material; exposing said substrate to a plasma; forming an amorphous silicon film on said insulator coating after exposing the substrate to said plasma; crystallizing said silicon film by photo annealing said silicon film and/or heat treating said silicon film in the temperature range of from 400 to 650xc2x0 C. or at a temperature not higher than the glass transition temperature of the substrate; selectively etching said silicon film; and establishing a channel forming region of a thin film transistor from the portion previously coated with the masking material.
Yet another embodiment according to the present invention provides a process for fabricating a semiconductor, which is characterized in that it comprises forming an amorphous silicon film on a substrate; forming, in intimate contact with the upper or the lower surface of said silicon film, a substance comprising an element which accelerates the crystallization of amorphous silicon by exerting a catalytic effect thereto; and crystallizing the silicon film by photo annealing the silicon film and/or heating the silicon film in the temperature range of from 400 to 650xc2x0 C. or at a temperature not higher than the glass transition temperature of the substrate. More specifically, the present process is accomplished by coating the surface of a base coating of the amorphous film with a solution containing water, an alcohol (either monohydric or polyhydric), a petroleum solvent (which may be a saturated or an unsaturated hydrocarbon), and the like, dissolved or dispersed therein an acetate or a nitrate, any type of carboxylate, or any other organic acid salt of an element which accelerates the crystallization. The coating may be otherwise provided on the upper surface of the amorphous coating film. The resulting amorphous coating is then subjected to heat treatment.
Still another embodiment according to the present invention provides a process for fabricating a semiconductor, characterized in that it comprises forming an insulator coating on a substrate; selectively coating said insulator coating with a masking material; exposing said substrate to a plasma or forming a coating of a substance containing an element which has a catalytic effect and accelerates the crystallization of the amorphous silicon; forming an amorphous silicon film on said insulator coating after subjecting the substrate to said previous step; crystallizing said silicon film by photo annealing said silicon film and/or heat treating said silicon film in the temperature range of from 400 to 650xc2x0 C. or at a temperature not higher than the glass transition temperature of the substrate; and selectively etching said silicon film.